1. Field of the Invention
The present invention generally relates to the design and control of internal combustion engines operating on two different fuels and capable of different modes of operations with different air fuel ratios.
2. Description of the Related Art
Internal combustion engines operate on the principle of igniting a mixture of air and gasoline (or other fuel) inside a cylinder to cause combustion within the cylinder where resulting released energy is converted to mechanical energy through the use of a piston inside the cylinder driving a crankshaft. Internal combustion engines are typically naturally aspirated meaning that air is drawn into the engine at atmospheric pressure. As a result of the combustion of the air fuel mixture within a cylinder of the engine, different types of unwanted toxic and pollutant gases are generated in the cylinder and pass through an exhaust system to a device commonly referred to as a catalytic converter.
The catalytic converter contains certain materials and catalysts to cause a chemical reaction to occur between said materials and the toxic, pollutant exhaust gases converting these harmful gases into less harmful emissions. Some of the harmful gases include carbon monoxide (CO), Hydrocarbons or volatile organic compounds, and Nitrogen oxides (referred to as NOx emissions). For efficient use of the catalytic converter, it is desirable that the air fuel mixture be maintained at the stoichiometric air fuel ratio of the fuel, which is used. That is, for exhaust gases resulting from a stoichiometric air fuel ratio, catalytic converters are best designed to convert these types of harmful exhaust gases into less harmful emissions. For an ideal combustion the amount of air and fuel used for combustion in a chamber of an engine is such that there is no residual oxygen or fuel remaining in the chamber after combustion, the particular air fuel ratio is referred to as the stoichiometric air fuel ratio. The stoichiometric air fuel ratio depends on the type of fuel used; for example, for gasoline the stoichiometric air fuel ratio is 14.7 lbs of air to 1 lb. of gasoline. The actual air fuel ratio of an engine is often expressed in terms of the stoichiometric air fuel ratio and the symbol λ (lambda) is typically used to denote the ratio between the actual ratio and the stoichiometric ratio. The mathematical expression for λ is thus:
                    λ        =                                            (                                                mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  air                                                  mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  fuel                                            )                        ⁢            actual                                              (                                                mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  air                                                  mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  fuel                                            )                        ⁢            stoichiometric                                              (        1        )            Based on the definition of λ above, internal combustion engines are ideally designed to operate at a λ=1 meaning that the air fuel ratio in an engine cylinder is maintained at the stoichiometric ratio or relatively close to the stoichiometric ratio to achieve the most reduction of harmful gases by a catalytic converter.
Referring to FIG. 1, there is shown a graph of λ as a function of time for a typical internal combustion gasoline engine. The upper boundary 102 of value 103 and lower boundary 104 of value 0.97 for a desirable value 108 of λ=1+/−0.03 are shown. The λ curve 106 is shown to be within the boundary values during operation of the engine. In many circumstances, the λ curve is outside of the boundaries resulting in relatively higher emissions of harmful gases into the environment because the catalytic converter cannot reduce these emissions as efficiently as the emissions resulting from a λ=1+/−0.03. When an engine is operated with a λ>1, the engine is said to be operated in a lean mode. When λ<1, the engine is said to be operated in a rich mode.